The present invention relates to techniques for improving radiograph images.
Since the discovery of x-rays and the early days of radiography (i.e., the exposure of x-ray shadowgrams on film), scatter has been recognized as a major contributing factor to image quality degradation. Scatter is the reemission of x-rays (called secondary rays) caused by the absorption of primary rays as illustrated in FIG. 1A. When the reemission of a secondary ray is in a different direction than the direction of the absorbed primary ray, the secondary ray blurs the shadowgram of the object.
Without scatter, the shadowgram sharpness is determined by the size of the x-ray source (called the focal spot), and the distances between the source, the object, and the image. Suitable detectors (e.g., films, intensifying screens, storage-phosphor plates, or solid-state detectors) are placed in the image plane to capture the shadowgram. When scatter is present, the shadowgram sharpness is reduced regardless of the resolution of the detector. Scatter is more pronounced in low-Z materials (such as soft tissue) than in high-Z materials (such as bone). Scatter is also more pronounced when a large x-ray cone beam is used rather than a small one. This is why radiographs of extremities (e.g., hands and legs) are much sharper than radiographs of the pelvis (i.e., a large area with significant amount of soft tissue).
A number of techniques have been suggested to suppress (or at least reduce) x-ray scatter. Some complex techniques involve sweeping a pencil beam or a fan beam of x-rays along with a secondary collimator (to block scatter). A simpler and commonly used technique invented by Gustave Bucky in 1913 involves using a cone beam of x-rays and placing a collimating grid 102 between the object (patient 104) and the detector 106 as shown in FIG. 1B. The purpose of the collimating grid is to absorb secondary rays and transmit primary rays.
Unfortunately, the collimating grid also absorbs a significant percentage of primary rays, and therefore the dose to the patient has to be increased accordingly so as to maintain the correct exposure level on the detector. The dose increase varies with the construction of the grid and its efficiency at reducing scatter, i.e., the greater the scatter reduction, the greater the dose increase. A “grid ratio” of 8:1 to 16:1 is not uncommon. This translates into a dose increase to the patient (i.e., the Bucky factor) of 4 to 6 times the dose necessary when no grid is used. In reality, not using a grid for certain exams (e.g., the pelvis) is not even an option since unblocked scatter would degrade the image quality below any acceptable level.
In view of the foregoing, it is clear that techniques for reducing the scatter in a radiograph are desirable.